Since the writer believes that an understanding of the physical history of the Permian period is fundamental to a sound classification of the rocks of this system, he has attempted to summarize the physical history of the Permian of the southern Mid-Continent with especial reference to the West Texas Permian basin. To this end seven paleogeographic maps are presented, drawn on the following horizons: latest Pennsylvanian (Thrifty), Wolfcamp (Coleman Junction), lower Leonard (Choza), San Andres (Blaine), middle Guadalupe (Grayburg), Castile and Salado, and Rustler. The regional correlations involved in making these maps are discussed in the text, and several supporting cross sections are illustrated. Some of the more prominent features of these maps are pointed out.

From the data on the areal extent of the seas, several curves have been constructed which show graphically many features of Permian history and throw much light on the physical basis for the classification of the rocks of this system. The text summarizes the history of the Permian in the southern Mid-Continent, and points out the relation of this study to the standard Permian section for North America set up by Adams et al.

The Wolfcamp epoch was initiated by the cessation of the Marathon-Arbuckle orogeny and the resultant constriction of the seas. This epoch was closed and the Leonard begun by the restriction of marine circulation and the beginning of large-scale evaporite deposition. These restricted seas are divided into marine, including the normal marine and vitasaline zones of Lang; saline, including Lang's penesaline, saline, and supersaline zones; and the brackish, including his brackish and fresh zones. The Leonard epoch ended with the retreat of the seas into the Delaware basin at the close of San Andres time. The Guadalupe was the epoch of reef-making and was brought to a close by the death of its greatest reef, the Capitan. The Ochoa commenced with a saline sea confined to the Delaware basin, and this epoch of restricted, highly saline seas was ended by the final Permian uplift which covered the evaporites with a blanket of red sands and shales.